01-12-2015
Quote:
Originally Posted by
Don Cragun
Not quite. It shows the size of the static data segment loaded when the process starts; it doesn't show the size of data allocated while the process is running (as in the 3 calls to malloc() in the given code snippet).
It is correct that the size command wont tell us anything about the runtime size of the heap allocated...but even accounting for malloc wont tell us how much data is allocated on the stack where function arguments and local variables are pushed...
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CHMEM(1) General Commands Manual CHMEM(1)
NAME
chmem - change memory allocation
SYNOPSIS
chmem [+] [-] [=] amount file
EXAMPLES
chmem =50000 a.out # Give a.out 50K of stack space
chmem -4000 a.out # Reduce the stack space by 4000 bytes
chmem +1000 file1 # Increase each stack by 1000 bytes
DESCRIPTION
When a program is loaded into memory, it is allocated enough memory for the text and data+bss segments, plus an area for the stack. Data
segment growth using malloc , brk , or sbrk eats up stack space from the low end. The amount of stack space to allocate is derived from a
field in the executable program's file header. If the combined stack and data segment growth exceeds the stack space allocated, the pro-
gram will be terminated.
It is therefore important to set the amount of stack space carefully. If too little is provided, the program may crash. If too much is
provided, memory will be wasted, and fewer programs will be able to fit in memory and run simultaneously. MINIX does not swap, so that
when memory is full, subsequent attempts to fork will fail. The compiler sets the stack space to the largest possible value (for the Intel
CPUs, 64K - text - data). For many programs, this value is far too large. Nonrecursive programs that do not call brk , sbrk , or malloc ,
and do not have any local arrays usually do not need more than 8K of stack space.
The chmem command changes the value of the header field that determines the stack allocation, and thus indirectly the total memory required
to run the program. The = option sets the stack size to a specific value; the + and - options increment and decrement the current value by
the indicated amount. The old and new stack sizes are printed.
SEE ALSO
install(1), brk(2).
CHMEM(1)